What does EL5 and EL6 means in term of packages?

I am new to CentOS and I am using it because Citrix XenServer is running on it.

[root@xen01 shm]# uname -a
Linux xen01 2.6.32.43-0.4.1.xs1.8.0.855.170800xen #1 SMP Mon Jul 21 05:12:35 EDT 2014 i686 i686 i386 GNU/Linux

[root@xen01 shm]# lsb_release -a
LSB Version:    :core-4.0-ia32:core-4.0-noarch
Distributor ID: XenServer
Description:    XenServer release 6.2.0-70446c (xenenterprise)
Release:    6.2.0-70446c
Codename:   xenenterprise

I installed apcupsd package, from http://ift.tt/1xEA4f9

But there was a new version the past year, and seems that RPM wasn't updated to 3.14.12.

I found this version however: http://ift.tt/1BKPaiJ

I would like to know what exactly EL5 and EL6 means in term of packages. The latter fails because of dependencies, but am I able to instal EL6 packages?

Thanks a lot.